Vinyl halide homopolymers, such as PVC, i.e. polyvinyl chloride, can be prepared in a wide variety of molecular weights, those having a higher molecular weight generally exhibiting better physical properties, such as toughness, than those polymers having a lower molecular weight. However, the higher the molecular weight of the polymer, the more difficult it is to process into final products. Thus, for example, in molding and extruding operations, the higher molecular weight vinyl halide polymers require more shear force and/or higher temperatures in order to flux the polymer as compared with lower molecular weight materials. The use of increased shear force and/or higher temperatures increases the internal temperature of the polymer. Therefore, since vinyl halide polymers are thermally unstable and degrade in the presence of excessive heat, this increase in the internal temperature of the polymer is highly disadvantageous. In addition, the use of increased shear forces requires the input of additional work energy, as compared with the processing of polymers of lower molecular weight, and this additional work input adds to the cost of processing the polymer.
In an attempt to obtain products characterized by the easier fluxing of lower average molecular weight vinyl halide polymers and the physical properties of the higher average molecular weight polymers, various blends of these materials have been formulated. However, these attempts to blend the two types of polymers during their fabrication have been fraught with difficulty. Thus, during processing, the low average molecular weight polymer fluxes first and, because of the poor heat transfer characteristics of vinyl halide polymers, the higher average molecular weight polymer is only partially fluxed. Furthermore, the shear forces applied during processing to affect flux are not applied to the higher average molecular weight materials as they are floating or suspended in the fluxed, low average molecular weight material and are, therefore, passed through the processing equipment without being fused. Products formed from such blends have noticeable imperfections called fish eyes which comprise gel particles of unfused, higher molecular weight polymer. These fish eyes are not only disadvantageous from an aesthetic point of view but also serve to extensively weaken the product. Needless to say, any polymer composition which must withstand severe impact forces such, for example, as films, sheets, tubes, bottles and the like must be substantially free from fish eyes. Since vinyl halide polymers are thermally degradable and become more unstable the longer they are worked under shear, it has been found impractical to subject such blends to high processing temperatures and/or to extended processing in order to overcome the problem of fish eyes. Such extended working is also economically impractical. In addition, fish eyes are a clear demonstration of imcompletely and improperly fused resin particles and are indicative of a commercially unacceptable product.
Thus, it is the object of this invention to provide a method for preparing vinyl halide homopolymers which exhibit improved processing characteristics without sacrificing physical properties. More particularly, it is the object of this invention to provide a method for preparing vinyl halide homopolymers which display physical properties approximating those ordinarily found in blends of high and low molecular weight polymers but which can be more easily processed and to provide final, formed objects which do not contain fish eyes. Various other objects and advantages of this invention will be apparent from a reading of the disclosure which follows hereinafter.